Hey fellas,
I'm starting to do some work on my mostly stock '95 with the intent of making it a pretty solid track car (Autocross. . . . NOT "drifting"). I'm pretty aware of all the good, lightweight wheels out there, but I'm trying to get some insight on the age-old 17" vs 18" dilemma.

I've done quite a bit of searching, but it's hard to get a definitive answer.

I'm going to lower the car with the pretty standard set of springs, which I think lower the front by .75" and the back by 1.5." Those numbers may be off a bit, but all the aftermarket springs I've seen tend to have the same spec as far as lowering is concerned.

The RZ has 17x8.5's if I am correct. my intent was to go with that size unless there's some combination that yields far better results. Most of the wheel threads here are along the lines of "let me see pictures." I'm more curious about handling and performance. let me know if you can shed any light on the topic.

Thanks.

 

From a purely functional stand-point, lighter is always better.
Most of the time, smaller rim diameters will mean a lighter tire and wheel combination.
Why not even drop down to a 16"?



-Ted

 

That is true, but obviously there are some aesthetic concerns here, though they aren't the chief concern.

I think, though, that larger wheels can be more stable in cornering. Maybe I'm wrong, but I think I heard something like this once.

 

There is no performance point of going to 18" wheels unless you want to run the widest tires available -- 285/30 18s. Otherwise, 17" should work just as good if not better. 17" wheels/tires will be lighter and cheaper.

17x9 with a high quality 255/40 tire will work great.

EDIT: Also, you will need to keep the stock 16" wheels for some classes, if you are auto-xing to be competitive. Something to keep in mind.

 

There is no definitive answer.

Wheel offset is critical then, that is, without going to a 2.5 inch spring. It limits your choices for wheel sizes. 17x10 on the front is probably not possible, 17x9 is probably okay. Rear, 17x10 should fit. Actually an 18 inch wheel diameter is easier to fit with the stock springs than the 17 inch.

Philosophy: It's better to first upgrade the driver, then upgrade the car. Invest in track time. It should be a progression as you gain experience on the track.

Example: You have a "mostly stock" car. Do all the reliability and safety mods so that it can at least run a full weekend at the track. Keep the stock wheels with a good street tire. Street tires give a great audible feedback to help you find the limits of the car. After you are comfortable with those, move to a stock sized DOT- R compound tire. They'll have more grip and are very quiet until you are at or beyond the limit, they'll also behave differently with varying track temps. Run those until you believe that you've comfortably reached the limits and need more grip. Then maybe move to a larger diameter/wider tire and wheel combo. Also remember that with increased speeds you're probably also stressing the braking system...

The downside to a wider tire is that, although you've increased lateral grip(corners), you've given up longitudinal grip(braking,accelerating). Contact patch area is constant, a wider tire just changes the shape of the contact area. A second downside is that the tire/wheel is heavier is slower to react to dynamic changes.

It's a very big subject and really can't be answered in one short post. Entire books have been written on the subject...

Mark

 

Thanks for the reply, art. A 17x8.5 front and 17x9 rear setup may work too, I'd guess, as a safe bet.

And thanks for the commentary, but I have most everything figured out. I'm an engineer by training and I enjoy working on cars more than driving them, though it's best to work on them and then drive 'em. Most total satisfaction.

Anyway, I'm not cutting my racing teeth on the Rx-7, since you seemed concerned about that. It's too fast to learn on. Right now I'm slowly fixing all the worn parts and making some replacements. This will take a while since I have a day job. Hopefully by the time I'm done I'll be ready to give it a go.

 

I ran 17x10 CCW with 275/40/17 Hossiers for the last autocross. It fits no problem.
My friend ran 18x10 CCW with 285/30/18 Victoracer. It also fit without problem.
You have to run in ASP or SM2 to do this. Can't change wheel diameter in SS.
One thing to consider is the overall diameter of wheel.
275/40/17 OD= 25.3" new
285/30/18 OD= 24.6" new

So that 0.7" difference translate to quite a few miles per hours difference, thus affecting your acceleration and top speed in 2nd gear. Generally we concern more on the bottom end acceleration. This is where we loose against corvettes.

That is why you see some people with NA cars, are using those 13x8 donuts, because they can bring the diameter to about 22 or 23 inches, then gain a lot of acceleration....

 

Read this, and then read it again. Good stuff

 

stock? or 2.5-inch outer diamter coil springs?

 

FWIW, I run this setup, with great success for road and track. I'm running Pirelli P6000s. Great tire for the road, but not great for track conditions. P-Zero Asimmetricos will be better.

See my pic in the sigs. They are 9 x 17, 45-mm offset SSR Integral A2s with 255/40-17s.

I have stock style H&R springs, stock R1 shocks, Eibach sway bars fr/rr, Tri-Point front sway bar mount reinforcer, M2 rear strut brace/harness bar, stock R1 front strut bar.

I run in SCCA Solo II SM2 class, and do many open track driving events @ various high speed road courses in the midwest.

 

Mine is JIC FLTA2
My friend is GAB with 2.5 GC Kit.

I think we can go faster in autox with the 18incher, because it allows the car to accelerate faster.

They should make 285/35/17 with OD of 24"

 

Is JIC FLTA2 a 2.5 inch spring kit?

 

Don't know....Did not measure it.
But it works with both the 17x10 and 18x10
No problem with the 18x10 at all.
17x10 a little problem, since Hossiers made the tires much wider that 10inch(>11inch). Thus when the suspension unloaded, the tire goes down, would rub near bottom of the spring a little bit. I had about 1/4 inch black mark from the tire on that purple spring, on the left side. Not on the right side. I think it happened only on one tight left hander turn that I spun on last autox. And oh ya, full lock left. I think 5mm spacer would fix that.
I don't consider this an issue. I would think Kumho don't have this problem.

 

Per Mark and Manny's posts, there are many ways to skin this cat, and there are alot of questions you need to ask yourself about specific classes or activities in which you'd like to run, your budget, etc...

But, one way to start is look at what brand of R-compound your looking at. Do you want to be able to drive home on them? Then that eliminates Hoosiers, and maybe even some Khumos, and the sizes they have available.

Drive-home-able tires like the A032 and RA1 come in limited sizes, and really arent doable in 18" for our car.

FWIW, Maxcooper uses the 285/30/18 size all around on his car, but he uses the BFG R1s that aren't even made anymore AFAIK.

The new Pilot sport Cups, and Pirelli offerings are slanted to 18" Porsche club sizes.

SO....

17" might be safer with regard to tire choices for track tires, depending on you budget/needs

 

Allright, I just got hit upside the head with this portion of the post.

The contact patch area is constant between my rear 225s and between, say some 315s? That's 90 millimeters in width. If that contact patch is the same, then what is the entire point in ever getting bigger tires? Or is this the issue that the x lbs of corner weight depresses the tire by y amount, creating contact patch area z. Between any dimension tire z will be the same? Doesn't seem to make total sense.

I thought the whole point of a larger tire was more rubber to the road = bigger contact patch =more grip in acceleration and more grip in braking.

 

You can increase grip with better compound tires for the conditions.

You can put more rubber to road contact patch if you increase aero down force, but grip won't necessarily increase if the rubber isn't soft enough to grip the pavement.

Wider wheels and tires make the contact patch more "landscape" shape, which is good for cornering grip, but like Mark says, the shape change is not necessarily good for braking and accelerating (better to have a "portrait" shape contact patch for this).

There's always a cost/benefit balance with any upgrade or change.

This is one reason why I decided that 255/40-17 tires on 9 x 17 wheels is the best balance between "landscape" and "portrait" contact patch change when cornering/braking/acclerating

Any wider than this, and you're only adding more weight, with no commensurate amount of grip.

Keep in mind our cars only weigh 2750 lbs wet, so contact patch is tough to make without aero aids (front splitter, and rear spoiler).

In contrast, heavy cars like Vipers and Vettes need wide wheels/tires because they make use of the large contact patch made by the weight transfers during braking/cornering/acclerating.

 

Gawd ... I feel like this topic has beaten to its ugly death so many times and yet it still raises its hydra-like head.

Many good points have been raised here and some erroneous ones as well. I won't go so far as to specifically point them out ... as I think experience is the best teacher in the world. So, use your better judgement and find out for yerself.

But at any rate, there are some questions that you need to ask yerself.
Primarily because there are no definite answers ... only ones that justify your ends.

1) What is your intent for the car? If you plan on making it "racecar", then don't sell yourself short. If you're looking to make a daily driver/capable racer (note NOT a racecar), then you will have to consider the necessary sacrifices.

2) What class do you want to run in? It makes a difference. SCCA classing is somewhat tricky in the wording, so be sure that whatever you do complies with the rules. The last thing you want is somebody protesting your times because you weren't familiar with the rules. Also, be familiar with your competition. Running bigger wheels and tires alone isn't going to make you competitive in ASP or SM2. There's a ton of work to be done just to make your car competitive ... let alone make YOU competitive. The best advice I can give you is "be realistic".

3) What are your budget expectations? I can tell you how to build the biggest, baddest, rock'em-sock'em auto-x/road race/wheel-to-wheel racer ... but if the price tag scares you, then what's the point? Lightweight wheels are EXPENSIVE. Hoosiers are EXPENSIVE. Bigger tires in general are EXPENSIVE. Am I making my point yet?

I'll tell you straight up that there's, relatively speaking, no difference between 17" and 18" wheels. You can always find a lighter weight version of one or the other ... just depends on how thick your wallet is. You can be competitive on either one. Really what you should be concerning yourself with is how much tire do you want to running on each end. Sizing and fitment will far more limit your thinking process than the advantages/disadvantages of each.

For auto-x, wider is ALWAYS better. The only time you want skinny(er) tires is if the course has standing water. This landscaping contact patch is bullshit. The f**kin' contact patch is bigger, just freaking admit it. Taller sidewalls will have better mid-corner grip due to the flex, but will have slower turn-in response. Shorter sidewalls will have more issues putting power down and be less forgiving at the limit, but will get the nose pointed in like all get-out. And wrt upsizing, a 285/30 18 has much better gearing (for auto-x) than a 275/40 and the 255/40. It doesn't stop there either ... you can fit 305', 315's, and 335's in their various 17" & 18" configurations. Just depends on much you want put into the car and how much benefit YOU will get out of it. I can go on at length about this stuff and what it takes to do it ... I don't think I have enough time or space to relate it all.

Anyhow, hope my tirade has provided you with enough insight.

-Don (having a bad day at work)

 

I have to disagree with this. A wider tire has a bigger contact patch; that means more available grip (all else equal). We're comparing "more" tire to "less" tire. "More" tire will have more grip in every direction than "less" tire, regardless of the shape you feel it's contact patch is in.

Contact patch area is constant how? A bigger tire (in diamter or width) is going to have more rubber on the road. Remember that rubber does not follow the nice force over area frictional equations that most materials do. The pressure between the contact patches and the road of two differently sized tires will not change without adding weight or aero to the chassis, but that doesn't mean the larger tire won't bring more grip. Tire traction is soley based on how many rubber molecules you can bring into contact with the road. You bring more rubber, you get more grip. Period.

 

Imagine a contact patch area the size of a 3 x 5 index card, bounded by a string. With wider wheels/tires, the string's length won't change, but the shape of the area bounded by the string will change. With wider wheels/tires, the contact shape will elongate to a more "landscape" shape, but will NOT increase in sqaure-inch area.

The contact patch's square-inch area is a function of the car's weight. Our cars are very light, and thus makes a small footprint. The only way to increase the square-inch area of the car's contact patch is to put more load on the tires. You can do this when the car is in motion with aerodynamic downforce (front air damn, rear spoiler).

Read the book, "Winning, a Racedriver's Handbook". The author was/is an SCCA club racer (don't recall the name).

 

I understand how the shape of the contact patch is different between a narrow and wide tire and I understand the shape of the patch can affect breakaway characteristics. Again the point I disagree with is that "wide" tires give up (or do not improve upon) forward grip compared to narrow tires.

Let's make this real simple. A car that puts lots of power to the ground and travels in only a straight line is perfect for this example; something like a drag car!

Am I to believe that wider tires of the same outer diameter do not produce more acceleration grip than narrower ones? Increased tire width is wasted on a dragster? You think a 5000 hp Top Fuel dragster would not love to have tires twice as wide? Bologna.

The variable that is being overlooked is that rubber is elastic. This "constant contact patch area" would be true if tires were made from a hard material. Tires are made from soft rubber which deforms around the microscopic road irregularities. Tires do not obey the idea of frictional force remaining constant due to a constant area. Tire grip is generated literally at the microscopic level. The fact that there is not as much weight on top of each rubber molecule in a wider tire is more than made up for by the fact that there are more rubber molecules touching the road surface. Interestingly you can also find that the rate at which the coefficient of friction rises due to download is not constant; the rate tapers off as download increases. This does not mean total traction doesn't increase with incrased download, but it does mean that a 50% increase in download does not equate to a 50% increase in grip; it may only translate to a 25% increase (or less!) in grip for instance. The relationship between grip and downforce is an increasing one, but not a constant one. The converse of this explains why all you need to increase grip is a wider tire.

In addition a wider tire is able to run a softer compound than the narrower tire and yet still deliver the same wear characteristics while running at a lower tire pressure. The wider tire can use lower inflation pressures (and thus increase its ability to deform against the road) because it has more area available to support the weight of the vehicle.

Wider tires do not improve cornering only. They improve every reaction between the vehicle and the road.

 

A dragster is a perfect example. As the tires spin, they deform and width becomes extremely narrow and the diameter is massive. Good for maximizing the longitudinal footprint and forward traction.

Let's look at an F1 car. Again, a perfect example for the opposite case. Extremely small diameter relative to its width. Good for maximizing the lateral footprint and sideways traction.

The tire supports the weight of the car. It deforms and produces some contact patch area. Unless you increase the weight of the car or loading on the tire, the contact patch area is constant.

This isn't a debate that's easily solved because there are some variables that are introduced with going to a wider tire other than just contact patch area.

It would be interesting to run an experiment: ink a tire, run over a piece of paper, measure contact patch area, repeat with wider tire. It may be a good way to see it graphically.

Mark

 

That is a very particular case for wrinkle wall drag tires which are designed to do that. I guess my Top Fuel example wasn't the best in that case; my fault.

Let's step away from wrinkle walls because they are a very different animal and stick with "regular" racing or street tires used on a drag car. A car with tires of the same diameter and yet twice as wide is not going to see an increase in grip under straight line acceleration?

 

Again, as Damon already pointed out ... you are assuming that the tire flex and the deformation are constant between different tire section widths. They are NOT constant. You can see the difference in your parking lot. After it rains and dries up a bit, take a look at the wet spots underneath the tires of .. say a Camaro with 275/40 17's and another with 245/50 16's. You tell me which spot is bigger.

 

This is easy. The truck is heavier than FD Rx7 by 2000 lbs, thus the commensurate increase in static contact patch of the truck vs the FD Rx7

 

Exactly. Probably the biggest variables in grip, is not the width/contact patch of the tire, but the rubber compound, its coefficient of friction, and the driver's ability to maximize available grip from the tire. With braking/accelerating, the driver will want to stay within 15% longitudinal slip. Any less, and you're not using all the available grip. More than 15% longitudinal slip, and you lock up or spin the tires. With lateral grip, the driver wants to use the tire's grip over the tire compound's range of usable slip angle. Racing slicks have a narrow range of usable slip angle (~5 degrees), with gripping falling off gradually. Street tires have a larger range of usable slip angle (~10 degrees), but the grip falls off pretty steeply past the usable slip angle range. The larger slip angle range is why I love using street tires at open track events. Street tires are much more entertaining, while racing slicks require tidier corner exits LOL